1. Field of the Invention
The present invention relates to a method of performing an electrical reject selection of integrated circuit devices having damaged passivation layers.
2. Descrition of the Related Art
In conventional integrated circuits, as shown in the partial cross-section of FIG. 1, interconnects 12 provide electrical connections between individual devices (not shown) on the integrated circuit and extend to the edges of the integrated circuit where enlarged regions called bonding pads 13 are provided. Metal "lead" wires 14 are attached to the bonding pads 13 in order to apply power, signals, and other electrical interconnections to the integrated circuit. The interconnects 12 are usually formed of 90%-100% pure aluminum and lead wires 14 are usually formed of gold, but may also be formed of aluminum. Lead wires 14 are attached to the bonding pads 13 by thermal compressive bonding which creates a bonded region 15.
The interconnects 12 and bonding pads 13 are covered and protected by passivation layer 16 formed of SiO.sub.2, doped SiO.sub.2, Si.sub.3 N.sub.4, or a combination of layers formed of these materials. Any defects 17 in the passivating layer 16, e.g., cracks, pinholes, and other attributes of the passivating layer 16 which expose the underlying interconnects 12 to contaminants and corroding agents may be fatal flaws in an integrated circuit. In particular, a portion 12a of interconnect 12 will be oxidized and/or otherwise corroded in a manner which creates a discontinuity in the interconnect 12 or converts portion 12a of the interconnect 12 to a nonconducting material, e.g., aluminum oxide or aluminum hydroxide. In either case the result is an integrated circuit which does not function properly since the affected interconnect 12 does not conduct electricity. Failures of integrated circuits caused by defects in the passivating layer may not occur until long after integrated circuit fabrication; however, it is desirable to identify integrated circuits which have the potential for fail during manufacture to avoid selling such devices.
Defects in the passivating layer 16 are difficult if not impossible to detect visually--any attempt to perform a meaningful visual inspection would be both time consuming, expensive, subjective as to pass/fail determination, and at best able to detect only a fraction of the defects. Such visual inspection requires the use of a microscope or other optical device having a limited field of view. A magnification of at least 500.times. is required for meaningful examination, and many defects cannot be detected without the aid of a scanning electron microscope. Nevertheless, conventionally only those integrated circuits having visible defects have been identified as rejected devices prior to the wafer sort and packaging steps.
Destructive tests using an acid etch followed by a visual inspection have been performed to test the integrity of the passivation layer in completed integrated circuit devices. These destructive tests destroy any device tested, and thus cannot be used on a wafer-wide basis. Instead, selected devices are tested to attempt to identify lots of bad devices.
Corrosion of the bonding pads is also a problem. Apertures 18 of approximately 16-20 mil.sup.2 are provided through portions of the passivating layer 16 corresponding to the center portions of the bonding pads 13 which are usually 20-25 mil.sup.2. Bonded region 15, however covers only 30-70% of the portion of bonding pad 13 exposed by aperture 18. Thus, a portion of each bonding pad 13 remains exposed after the leads 14 are attached thereto. The regions of aperture 18 where bonded region 15 does not cover bonding pad 13 the aluminum bonding pad 13 is subject to oxidation, or other corrosion. This oxidation or corrosion transforms aluminum bonding pad 13 into a non-conducting material. If the oxidation of bonding pad 13 occurs in region l3b, it will cause increased resistance between the bonding pad 13 and interconnect 12, and in the worst case an open circuit between bonding pad 13 and the remaining portion of interconnect 12.
In the case where the passivation layer 16 is doped SiO.sub.2, moisture which enters the integrated circuit package combines with phosphorous, which is present as a dopant in the passivating layers 16, to form phosphoric acid which reacts with the exposed aluminum converting the aluminum to aluminum oxide or aluminum hydroxide. This phenomenon is facilitated by the direct contact between the exposed portion of bonding pad 13, particularly portion l3b, and passivating layer 16.
Other corrosion sources may be associated with the packaging material of an integrated circuit. Generally, the packaging is a plastic material which may have components which cause corrosion. In addition, other sources of corrosive materials may exist in the integrated circuit itself or may occur as the result of fabrication or packaging processes.